As outside surface materials of sanitary materials, typically backsheets of disposable diapers, sanitary napkins and the like, moisture-permeable single films have been used in the past, or air-through nonwoven fabrics or short fiber nonwoven fabrics such as point-bonded nonwoven fabrics have been used in combination with moisture-permeable films in the past. These conventional materials, however, have individual problems. For example, the moisture-permeable single films have a bad hand feeling though they are free from fluffing, the air-through nonwoven fabrics have problems in strength and fluffing resistance though they are soft, and the short fiber nonwoven fabrics have problems in softness and strength though they are excellent in economical efficiency and fluffing resistance.
With regard to use of the nonwoven fabrics for backsheets, it is disclosed in Japanese Patent Laid-Open Publication No. 972/1999 (patent document 1) to use a nonwoven fabric including a spunbonded nonwoven fabric together with a moisture-permeable film. In the existing circumstances, however, balance between fluffing resistance as a main property and other necessary properties such as strength and softness has not been studied yet similarly to the short fiber nonwoven fabrics. In the case where a nonwoven fabric is used as a surface material that is always exposed to friction, such as a backsheet, fluffing resistance is an important property. For example, when the nonwoven fabric is used for a disposable diaper, fluffing occurs and a fluff ball is formed, so that there is danger that an infant puts the fluff ball in the mouth, and besides, a problem of very bad appearance occurs. When the nonwoven fabric is used as a backsheet of a sanitary product, softness also becomes important. The reason is that there is a direct relation between softness and a feeling in use that is important for the user. Also in order to improve the touch for a person who wears the sanitary product, a helper, a nurse and the like, softness is necessary.
For example, softness of the nonwoven fabrics is described in Japanese Patent Laid-Open Publication No. 288260/1988 (patent document 2) or Japanese Patent Laid-Open Publication No. 280267/1998 (patent document 3). Further, with regard to strength of the spunbonded nonwoven fabrics, improvements have been made as disclosed in Japanese Patent Laid-Open Publication No. 292256/1998 (patent document 4). These improvements, however, are not intended for the surface materials such as backsheets, and studies including balance between fluffing resistance and other properties have not been made yet.
In the case where propylene-based polymer spunbonded nonwoven fabrics are provided for backsheet applications, examples of conventional methods to obtain softness include a method of carrying out a post processing step and a method of weakening compression bonding of a hot embossing roll. The former method, however, is inferior in productivity and economical efficiency, while in the latter method, fluffing resistance is lowered though softness is obtained. If basis weight is merely decreased in order to obtain both of fluffing resistance and softness, strength of the nonwoven fabric is lowered, and therefore, the degree of freedom in design of products such as diapers is restricted.
An attempt to obtain softness by decreasing fineness has been made, but the fiber lengths of fibers that have not been compression bonded are increased, and from this, lowering of fluffing resistance is presumed. Then, if severe compression-bonding temperature or contact pressure is applied in order to make the compression bond portion stronger, heat history of the non-compression bond portion is also increased, and hence, there occur problems such that the fibers lose softness, and breakage takes place in the compression bond portion, that is, an excess compression-bonding phenomenon such as pinhole takes place.
In order to decrease fineness, there are a method of increasing a spinning rate and a method of decreasing a discharge rate, and it is well known that in the case of polypropylene, the non-crystalline part or the loosely crystalline part, that is considered to participate in the compression bonding, is apt to be produced rather by decreasing a spinning rate. However, in order to produce a nonwoven fabric having a small fineness by this method, decrease of the discharge rate and decrease of the spinning rate become necessary, and in addition, lowering of the resin viscosity also becomes necessary to enhance fluidity. In such a method, therefore, there are problems such as deterioration of spinning property, lowering of productivity and lowering of physical properties.
By the way, in Japanese Patent Laid-Open Publication No. 105832/2002 (patent document 5), a spunbonded nonwoven fabric requiring no post processing, having excellent fluffing resistance and also having softness and strength is described. In the patent document 5, however, polypropylene having a relatively low melt flow rate is used as a raw material, and therefore, lowering of productivity attributable to occurrence of thread breakage is presumed. Moreover, it cannot be said that this spunbonded nonwoven fabric has sufficient softness, and therefore, further improvement is required.    Patent document 1: Japanese Patent Laid-Open Publication No. 972/1999    Patent document 2: Japanese Patent Laid-Open Publication No. 288260/1988    Patent document 3: Japanese Patent Laid-Open Publication No. 280267/1998    Patent document 4: Japanese Patent Laid-Open Publication No. 292256/1998    Patent document 5: Japanese Patent Laid-Open Publication No. 105832/2002